AP BiologyCommon mistakesLight reactions make ATP. The Calvin cycle uses CO2.

Light reactions make ATP. The Calvin cycle uses CO2.

  1. The idea
  2. Predict
  3. See it
  4. Why
  5. Check

The idea

“The Calvin cycle makes ATP, and the light reactions use the CO2.”

The two stages trade molecules back and forth, so it's easy to mix up which one makes what.

The short version

The mistake: The Calvin cycle makes ATP, and the light reactions use the CO2.

What's actually true: The light reactions use light and water to make ATP and NADPH. The Calvin cycle spends those to turn CO2 into sugar.

  1. The light reactions happen in the thylakoid membranes. They use light to split water, which releases O2, and they make ATP and NADPH.
  2. The Calvin cycle happens in the stroma. The enzyme rubisco attaches CO2 to a sugar called RuBP, and the cycle spends ATP and NADPH to make sugar.
  3. The cycle makes no ATP. ADP and NADP+ return to the thylakoids to be recharged.

On the exam: The light reactions make ATP from light and water. The Calvin cycle spends it to turn CO2 into sugar.

Every common AP Biology mistake